In modern (heart) vessel surgery (Percutaneous Transluminal (Coronary) Angioplasty PT(C)A expanding plastics—so-called stents—are inserted) to an increasing extent intravascularly into stenotized vessel sections. This involves implants usually in the form of thin wire meshes (wire diameter only a few hundredths of a millimeter) in rare cases also in the form of small metal or plastic tubes, which prevent a renewed narrowing of the vessel or even worse a closing-off of the vessel at the relevant point by supporting the vessel wall to hold it open.
A stent is introduced either after or during an artificial expansion process e.g. by means of an ablation laser, thrombectomy catheter, etc. mostly however with a balloon catheter. The balloon catheter is introduced via a guidance wire and pumped up with water mixed with contrast media or just with air in order to expand the point in the vessel concerned so that the stent can be introduced subsequent to the removal of the balloon catheter. However if the vessel is already too weak for this, a (new) balloon catheter is introduced which functions in its slightly inflated state as a stent support, in that the stent is pushed over outer membrane of the balloon and is inserted held by the balloon into the narrowed area of the vessel, i.e. implanted. The balloon is deflated and withdrawn. Compared to a surgically much more complicated bypass operation, a catheter-based stent implant represents a quicker, more efficient and also less hazardous implantation method.
An absolute requirement in such cases however is a precise and very good visualization of the stent in the surrounding tissue during and after the implantation, in order to enable both the position and also the instantaneous state (e.g. the degree of unfolding) to be assessed.
The visualizing can be undertaken with a wide diversity of imaging modalities such as computer tomography, fluoroscopy, sonography or magnetic resonance tomography for example, on which however high demands are imposed and which at the current time have still not been satisfactorily resolved.
There are various reasons for the high demands imposed on the angiographic imaging technologies, for example because—as already mentioned—a stent typically consists of a very thin wire mesh (stent mesh diameter in the region of a few hundredths of a millimeter) and a movement within the framework of the wire diameter (during the acquisition of a snapshot) is sufficient to make an image of the stent unusable.
It is also problematic if the thickness of the body tissue to be imaged by x-rays for example is comparatively large and if highly absorbent structures cast shadows over the stent.
Also necessary for good visibility is a sufficient resolution of the object with a sufficiently large quantum flow density where x-ray radiation is used as the imaging technique. As a rule the number of verifiable quantas per detector surface from the x-ray source and the image acquired is not sufficient, since the time Δt of a recording process (shot time) must be kept short because of the imprecisions of movement as the time t increases and in addition the power of the x-ray tube is limited. In addition the distance between patient and x-ray detector must be minimized because of the increasingly imprecise movements which in its turn leads to the object of interest (the filigree vessel implant) appearing as an image on a comparatively small area of the detector surface. If a Flat-Panel Detector (FPD) is additionally used as an x-ray detector, converting the x-rays which have penetrated the body directly into an electrical signal (direct converter) under some circumstances its restricted local resolution also comes into play.
All these factors show that an artifact-free visualization, localization, identification and status evaluation of filigree vessel implants still poses a problem, at least with angiographic fluoroscopy.
The current practice is thus either to be satisfied with the result of a standard angiographic examination (as a rule these are 2D projection images, coronal, sagittal or axial 2D-sectional images in the x-ray CT imaging or 3D-representations of the segmented vessel segments incl. embedded vessel implants in the magnetic resonance tomographic imaging) or an attempt is made to make the stent visible through “inter-image postprocessing”.
This method represents an arithmetical averaging (over time) of the image data of a number of consecutive similar images (e.g. angiocardiograms of one and the same section of an image). Since objects specifically visualized in PTCA by means of CT or MR angiographic imaging (e.g. balloon catheters and vessel implants with guide wire, guide tube etc.) move very fast because of the movement of the heart, a movement detection is indispensable. The latter enables the image data acquired by means of CT or MRT angiography to detect and evaluate the respective movement sequences in real time.
It is proposed in the three publications 03/043 516 A2, 04/051 572 A2 and 05/104 951 A1 that two markings of the balloon catheter or catheters used be employed as registration markers of the object of interest (vessel implant, e.g. stent). Balloon catheters have a radio-opaque and MRT-active marking at both their proximal and also at their distal end (balloon marker MP, MD) which thus delimit the balloon catheter axially. If the respective markers are registered in consecutive images, the objects of interest of these images can also be registered, i.e. moved, turned of stretched (scaled) so that these come lie on top of each other as closely as possible and can finally be averaged.
The averaging obliterates the background and in particular greatly reduces the noise, so that the object increases in contrast. However this process requires the basic assumption that the actual object of interest (the stent) moves exactly and is linearly scaled like the respective imaging of the balloon marker. This is in fact the case if a rigid connection between the stent and the balloon is assumed, but this does not represent the general case. In general the balloon does not remain in the inflated state after the placement of the stent, since this would result in a blocking of the blood flow or because of the contrast media filling would impede the visibility the stent meshes.
In actual fact there are frequently small relative movements of the stent in relation to the balloon markers. With longer stents in particular the result can be a partial separation of the balloon from the stent on one side caused by the process of deflating the balloon. Often the relative movement then represents a movement of the marker or markers in the radial direction relative to the stent. As already mentioned, movements in the order of magnitude of the stent mesh diameter (appr. 0.01 mm) are sufficient to make an image of the stent unusable or to prevent it.